These circuits are really not hard to build. I've actually built them before. The only problem with doing this to such a heavy current capable battery is the fact that unless you can seriously load it up to drain the current until discharged, you are going to either build a programmable load or use selectable fixed loads. Either way those loads will have to be able to accommodate lots of heat and dissipate it as well... Otherwise a smaller load will take forever to accomplish the discharge and recharge cycling results... The R/C communities have these listed everywhere and are programmable. But they are for a lot smaller current capability battery types. But the entire concept is already available except for smaller batteries. And the only issue with them would be the loads they apply!
The problem in the boating world is meeting the price point for a device that the typical boater would be willing to pay.
The vast majority of batteries get placed in the boat, and remain there unused and frozen for 6+ months,
Water levels are never checked, and it is accepted that they will fail and require replacement every few years. "Why bother testing if I have a 5 year warranty? "
If the batteries were stored in a warm area for the winter and the water level was checked at least once a year they would last at least twice as long.
Many boaters complain of the need to even charge batteries between trips because it is too much trouble.
Volts and Amps are commonly not understood. Explaining Ampere-Hours will be like explaining Differential Calculus.
The boating community is a filtered cross section of the population.
Most of our neighbors think we are rich because we even own a boat.
A small amount of households even have a battery charger, even less will view a Tester as something useful.
An automatic Large Capacity Battery tester will have a limited but real market.
The Retail/Consumer market won't play a large part of it.
Tow-motor and Golf Cart users might be more receptive if it is marked as a device that will save them money instead of costing money.
A $200 tester is a good deal if prevents replacing $1000 of batteries that are actually still good.
Do you see your market a a few hundred units or a few hundred thousand?
Just seeing what the boat community needs to see if it's worth my time to pursue the idea.